In the Kraft pulp production process, a fibrous material, most commonly wood chips, is broken down into pulp in a digester under pressure in a steam-heated aqueous solution of sodium hydroxide and sodium sulphide, called white liquor. After cooking in the digester, the pulp is separated from the residual liquid called black liquor. Black liquor is an aqueous solution containing wood lignins, other organic material, and inorganic compounds oxidized in the digester during the cooking process. It is dried in the evaporation plant to 55-85% dry solids concentration (concentrated) and then black liquor 2 is sprayed (3) into the furnace 1 of the recovery boiler, and burned (in a recovery boiler) to recover cooking chemicals (FIG. 1), and generate steam, which is used in the pulp mill for power generation, for pulp cooking and drying, for black liquor drying, and for other energy requirements.
The inorganic material in black liquor is recovered in the recovery boiler for reuse in the cooking process. This recovery requires special, reducing atmosphere in the lower furnace. Typically this is achieved by creating a char bed 4 on the floor 10 of the furnace. The shape and size of the char bed depend on boiler design, but it can be 1-2 meters high at the highest spot, calculated from the smelt overflow height 15. The inorganics are taken out from the recovery boiler furnace as molten smelt 16a and 16b, the main compounds in smelt being typically Na2CO3 and Na2S, with smaller smaller amounts of potassium based compounds. Smaller amount of non-process elements are also flowing out from the furnace in the smelt.
Liquor is sprayed into the furnace from several locations 3, which are called ports. The ports are typically located on one level, called liquor feed level, but there can be also more levels to meet special requirements. When liquor is sprayed into the furnace, it heats up due to hot atmosphere, which result in drying and in pyrolysis. In the pyrolysis phase the organic structure of black liquor is destroyed; part of the material will end as pyrolysis gas into the furnace atmosphere, and part of the material passes further as char. Both material streams ignite and burn, until the organic material has been consumed. Only a very small part of the original organic material in black liquor leaves the furnace as unburned in modern recovery boilers. Depending on the original droplet size, the char burns totally in flight, or end into the char bed 4, and onto furnace walls. In modern recovery boilers drying, pyrolysis and combustion on furnace walls should be minimized. The char bed is formed of burning liquor droplets 12, burning char and inorganic material, in which sulphur compounds are reacting from oxidized form to reduced form. This reduction requires the presence of carbon, and thus the char bed control is essential for achieving good reduction efficiency. The reduction efficiency expresses which share of total sulphur in smelt, flowing from the furnace 16a, 16b, is in reduced form, i.e. as Na2S+K2S. Typically this is over 90%. When reduction is good the reduction efficiency is over 95-96%.
Small liquor droplets are also generated during liquor spraying, and these droplets 13 dry, pyrolyze and burn in flight. The droplets, finally entering the floor area of the furnace, tend to contain oxidized sulphur due to the combustion atmosphere in the upper furnace. Then again carbon is needed for sulphur reduction. The good total reduction requires good carbon coverage over the whole floor. The reactions between carbon and oxidized sulphur, the most important of which is, Na2SO4, are strongly temperature-dependent, and require energy. Thus only a relatively thin surface layer 14 on the surface of the char bed 4 is active, which means that the char bed does not have to be high. Controlling possibilities and characteristics of liquor spraying and different combustion air feeds, together with the reduction characteristics, dictate in practice the shape of the char bed. If the bed grows too big, there is a risk of bed falling into airports, typically into primary airports, and a risk of smelt escaping via smelt spouts into the dissolving tank or into dissolving tanks.
An effective burning process requires that the char bed can be controlled reliably. Thus a need to monitor and control the size and shape of the char bed in a Kraft recovery system has been recognized for many years; however, no reliable technique for controlling the char bed automatically has yet been available.
Gas temperatures in the furnace range typically from 100-200 degree C. which is the temperature of incoming air and liquor to 1200-1400 degree C. in the hottest areas of the furnace, which is for instance the area, where tertiary air is fed into the furnace, or where final combustion takes place. On the char bed surface the temperature is typically 900-1200 degree C. The temperature of the smelt exiting the furnace is typically 800-900 degree C. The clean walls 8 of the furnace have a temperature of 250-400 degree C., depending on the pressure of the boiler and on the observation point; tube or the fin between the tubes, which has higher temperature than the tube, inside which evaporating water flows, cooling the furnace walls and generating the main portion of steam for superheating in super heaters. Deposition takes typically place on furnace walls, which raises the surface temperature of the deposit closer to temperatures in the gas phase and in the char bed.
All the surfaces radiate thermal radiation. This radiation is basically continuous, but changes in radiation properties, such as emissivity, as the function of temperature causes that the radiation intensity distribution does not follow the Planck's law. Naturally, when the dependency of the radiation properties as the function of the temperature and composition is known, proper correction factors can be generated to fit the measured intensities on several wavelengths, to estimate the surface temperature of the radiating surface.
Gases, liquids and solids in the furnace gas atmosphere radiate as well, but this radiation is concentrated, at least partly, to spectrums; and there may be areas in the wavelengths, where radiation or absorption is weak. These windows are potential for imaging the char bed. The small particles in the furnace radiate and scatter incoming electromagnetic radiation, complicating the system. Thus the electromagnetic radiation phenomena in the furnace are very complex. The key factor to be able to image the char bed from the hot gas atmosphere around, with vapours and particles, is to receive electromagnetic radiation information from the char bed, which is not excessively influenced by the surrounding atmosphere.
It is known to use a TV camera mounted in a special port or into an air inlet port to monitor the bed, i.e. the TV camera continuously scans electromagnetic information from the bed and a TV set provides a picture in the control room so that the operator may use this picture to control the furnace. The detector of this type camera operates today typically around 1.7 micro meter wavelength.
Such a means for monitoring the bed height and shape with TV cameras and having the capability of automatically reacting when the bed deviates beyond the limit from a preset height and/or shape and to control furnace operating parameters to maintain the bed at the required height and/or shape has been disclosed in CA 1166842. The recovery boiler is provided with ports to mount the TV cameras, or the cameras may be mounted in selected airports. The signal from these cameras is carried via lines to a television monitor that visually displays the picture of the bed taken by each of the cameras on a monitoring screen in the control room. The signal is also carried to a video image processor which digitizes the images sensed by each of the cameras coding each point of each frame based on shade or greyness or brightness to permit an analyzer built into the image processor to discriminate between the different shades and thereby obtain the interference between the char bed and the surrounding atmosphere. In this manner the location of the interface and thus the outline of the bed are determined.
While a camera responsive to visible radiation may be used, the fume particles and gaseous radiation cause problems in the visible region and the intensity of infrared emissions from the process area will be greater than emissions in the visible portion of the spectrum. Further, environmental factors related to the process environment may interfere with infrared emissions less than visible emissions. For these reasons, an infrared camera, as disclosed for example in U.S. Pat. No. 5,219,226, may be used to produce a video image representative of the intensity of received infrared radiation.
An disadvantage of the prior art solutions using conventional TV or IR (infrared) cameras is that they are only able to form a plane (2D) view of the char bed which cannot provide a reliable image for control purposes.
Some efforts have been made to obtain a more reliable image of the char bed. JP-A-61130725 discloses a char bed monitoring device wherein a TV camera and image processing device are provided for picking up the char bed and for forming a three dimensional image of the char bed by means of an image signal. However, in this JP publication the TV camera produces a conventional 2D image signal, and the 3D image is afterwards achieved by image data processing with help of the plane view information. This requires a lot of data processing capacity and is not a suitable system for forming a 3D image of the char bed for control purposes.
A further disadvantage of the prior art solutions is that they operate only at a certain wavelength range. The prior art systems thus use optical filters to limit the wavelength of electromagnetic radiation transmitted from the bed to be imaged, typically to wavelengths greater than 1 micro meter. Typically the filter limits the transmitted light to a narrow band, as disclosed for example in U.S. Re. 33,857.